When first starting to pump, the "burn" discomfort is caused by skin stretching ... but, fortunately, skin stretches relatively quickly. My advice is to put a very thin coating of lube (Albolene is my favorite) on your cock before getting in the tube ... then increase the vacuum to a point of tolerable burn at each session. Over days or weeks, the skin will stretch and allow higher suction. Even today, I start every session with a few minutes of stretch at 3-5 psi, then a few more at 8-10 psi, etc. Over time, the skin is conditioned to the stretching, and the ultimate goal of expanding the size/capacity of the two internal "corpus cavernosa" starts ... they're the two internal chambers that engorge with blood during an erection.